This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Immunization with recombinant replication-defective herpes simplex virus (HSV) d106 vector-based and DNA vector-based SIV vaccines expressing Gag, Env, Rev, Tat, and Nef was tested for protective efficacy against homologous high-dose, intra-rectal SIVmac239 challenge. Thirty rhesus macaques received two primary inoculations at 0 and 4 weeks, two booster inoculations at 12 and 24 weeks, and were challenged with SIVmac239 at 36 weeks. The prime-boost immunization regimens (n=6 per group) consisted of DNA prime-DNA boost, DNA prime-HSV boost, HSV prime-DNA boost, HSV prime-HSV boost, and a control HSVd106 vector group. Following challenge, peak viremia levels were roughly one-log lower in all four groups of vaccinated macaques compared to the control macaques;the maximum reduction (e30-fold) was observed with the two heterologous prime-boost immunization regimens. At later time-points, the HSV prime-DNA boost immunization regimen showed the maximal protective efficacy as evidenced by 100 percent survival and 2.3 log reduction in set-point viremia at one year post challenge. A vaccine-mediated protective effect was also discernable with regards to the extent of depletion of CD4-positive T lymphocytes in the gut mucosa. Vaccinated and control macaques showed comparable degree of gut CD4-positive T lymphocyte depletion at two weeks post challenge. However, at 12 weeks post challenge a significantly higher frequency of CD4-positive T lymphocytes in the gut mucosa was observed in the HSV prime-DNA boost group of vaccinated macaques compared to control macaques. The superior protective efficacy of the HSV prime-DNA boost immunization regimen did not appear to be predicted by the cellular immune responses induced during the vaccination phase since the highest magnitude of interferon-gamma ELISPOT responses were detected in the DNA prime-HSV boost group of macaques. The ability of HSV prime-DNA boost to confer partial protection against pathogenic SIVmac239 challenge makes it a promising vaccine approach against AIDS.